Inherent drawbacks of two stroke engines are high emission levels and poor fuel economy due to short-circuit loss of fuel and air charge during the scavenging process. One drawback of the simple two-stroke engine is a loss of a portion of the fresh unburned fuel charge from the cylinder during the scavenging process. In the two-stroke engine, the homogeneous charge enters the cylinder through transfer ports during the scavenging process, when the exhaust port is also open. As such, some of the charge escapes through the exhaust port leading to high levels of hydrocarbons (HC) in the tailpipe. This leads to the poor fuel economy and high emission of unburned hydrocarbon, thus, rendering the simple two stroke engine difficult to comply with increasingly stringent governmental pollution restrictions. This drawback can be relieved by separating the scavenging of the cylinder, with fresh air, from the charging of the cylinder, with fuel. This separation can be achieved by having a buffer medium of air between the fresh charge and the burnt gas, during the scavenging process.
Several concepts and technologies have been proposed or tried to circumvent the short-circuit loss of fresh charge. Among these techniques are direct or indirect fuel injections, stratified scavenging, air-head, air assisted fuel injection, and compressed wave injection. Most of these technologies are either complex, expensive or need more parts. The fuel injection technology is not economical for small engines but air-head scavenging and stratified scavenging are promising.
Air-head scavenging systems disclosed in U.S. Pat. Nos. 4,821,787, 6,112,708, and 6,367,432 describe reed valve controlled air passages in air-head scavenged two-stroke engines. The use of reed valves increases the cost.
U.S. Pat. Nos. 7,363,888, 6,973,899, 7,025,021, 6,895,910, 6,289,856, and 6,497,204 describe piston controlled air head scavenging. However, the location of the ports with respect to the crankshaft do not meet the configuration necessary to have the exhaust port in line with the crankshaft as packaged by, for example, Echo brand chainsaw. Secondly the inlet of air requires dual ports to supply air to transfer passages on either side of the exhaust port.
U.S. Pat. No. 7,331,315, and Application 2006243230, describe a fuel injected stratified engine. However, there are several drawbacks of those fuel injection systems. First, one hundred percent of the air goes through the transfer passages during the induction process and the same transfer passages are then used to transfer the charge from the crankcase to the combustion chamber during the gas exchange process. Secondly, the transfer passage ports are likely to be very large and the fuel may stick to walls in transfer passages, at least during cold start, and some of it may be lost into exhaust, which increases HC emission.
It is desirable to have a simple two-stroke engine with fewer parts and that is easy to manufacture and assemble. It is also desirable to have a piston that can be die cast for low cost manufacturability.
In most engines, fuel is mixed with air using a simple carburetor. However, among the disadvantages of the carburetor systems are that they need a manual choke and do not compensate for variation in ambient and operating temperatures. Thus the fuel consumption is higher and hence brake specific emission is also higher. Also, the conventional carburetors in small engines have built in fuel pumps that depend on the pulsation of crankcase pressure. There are more advanced electronic fuel systems commonly used in automobiles and some small engines. For example U.S. Pat. Nos. 7,331,315, and 7,536,983, and PCT US2007/074982 describe electronic fuel injection systems for small two-stroke engines, which have fuel pumps that depend on engine pulses for pumping the fuel at a certain pressure, but can become unreliable as they entirely depend on crankcase pulses. For instance, the crankcase pulses could be affected by the blow down of burnt gases into the crankcase chamber and as such pulse pump could be unreliable. Some engines use electrical or mechanical pumps for delivering fuel at a higher pressure to the injector. Secondly they use gasoline as fuel. In U.S. Pat. No. 6,609,509 the fuel used is LPG (liquefied petroleum gas), however, the system is more of a carburetor type than electronically controlled injection system.